1. Field of the Invention PA1 2. Description of the Prior Art
The present invention is directed to an improved friction welding apparatus and related apparatus for its use and application in the field. More specifically, the present invention is directed to a portable friction welding apparatus operable via low pressure air sources commonly found in industrial settings, and methods for its use.
In many settings, and especially industrial settings, it is often desirable to attach two members or workpieces via a high strength, fine grain weld. Such a weld is usually performed via a conventional arc or open flame welding procedure. However, in applications where volatile or combustible gases are present, it is not usually possible to use an arc or open flame welding procedure due to the attendant danger of fire or explosion.
One of the solutions proposed to address the above problem includes the use of a friction weld procedure. The friction welding process relies on heat generation between rubbing surfaces to provide a material flux which may be forged to produce an integral bond between the two surfaces. There are generally two recognized methods of supplying energy to form a friction weld: direct drive friction welding, sometimes referred to as "conventional" friction welding, and inertial welding.
In conventional frictional welding, one of the workpieces is attached to a motor-driven unit and rotated at a predetermined, constant speed, while the other member is maintained in a fixed, stationary orientation. When the appropriate rotational speed is reached, the two workpieces are brought together and an axial force is applied. Heat is generated as a result of the friction generated by interface of the respective surfaces, which interface continues for a predetermined time or until a preset amount of upset takes place. Thereafter, the rotational driving force is discontinued and the rotation of the workpiece is stopped. The axial force between the two members is maintained or increased, however, for a predetermined period of time to finalize the weld. The weld product result from a conventional friction weld process is characterized by a narrow heat affected zone, the presence of plastically deformed material around the weld, and the absence of a fusion zone.
A number of disadvantages exist with the direct drive or conventional friction welding process. One such disadvantage is the overall bulk of such a system, since a typical direct drive friction weld apparatus is usually both large and cumbersome. Moreover, conventional friction welding apparatus also typically include complex electronic controls for controlling the different forces which must be applied and for controlling the drive means in a selective manner to monitor relative rotation of the workpieces. In rigorous applications such as those presented in the industrial environment, such electronic controls are often prone to failure. Moreover, the presence of electronic controls requires the presence of an electronic power source which is often times unavailable in the industrial setting.
Yet another problem which occurs with conventional friction welding arises when the two workpieces are initially brought together. At this stage, there is significant initial friction between the workpieces and, therefore, a considerable increase in the energy required to overcome the initial friction. This problem is further complicated in welding rotational workpieces to stationary workpieces due to wide variations in frictional torque throughout the weld cycle. On initial contact of the welding surfaces, there is a relatively high frictional torque which is shortly followed by a requirement for inertial energy which persists until a flux of hot metal is established. However, this energy requirement is temporary in nature and ceases after the resistive torque has been overcome. When the flux is established, the resistive torque falls to a level during the "burn-off" and "upset" phases which may typically be as low as some twenty-five percent of the initial peak torque. During this phase, axial pressure is maintained and the contact surface of both members are carbonized, in the instance of a carbon steel, thereby adding to the flux. This upset phase continues until the driving torque is removed after which time the flux cools, the weld fuses and the resistive torque increases.
The above-noted problems have been addressed in the prior art by the development of drive motors capable of supplying sufficient torque to overcome initial friction forces. Such a drive motor is generally acceptable in relatively stationary friction welding apparatus. However, this proposed use of high power drive motors, due to their large power requirements and weight, are unacceptable to the design of a portable friction weld apparatus.
Inertia friction welding was developed to address the above disadvantages of prior art "conventional" friction welding techniques. Contrasted with conventional friction welding, in inertial friction welding the speed of the rotating workpiece continuously decreases during the friction stages of the procedure. In inertial friction welding, the rotating workpiece is coupled to a flywheel which is accelerated to a predetermined rotational speed. During the weld process, the drive motor is disengaged and the workpieces are forced together in an axial direction. This axial force causes the forging surfaces to rub together under pressure. The kinetic energy stored in the rotating flywheel is ultimately dissipated as heat as a result of friction between the workpieces. As a result of such friction, the speed of the flywheel decreases until stoppage during which time the axial force may be increased or maintained. The total time for the wheel to come to rest depends on the average rate at which the energy is being removed and converted to heat.
Three variables are presented by the inertial welding technique. These include the movement of the inertia of the flywheel, the initial flywheel speed, and the axial pressure between the workpieces. The first two variables dictate the total amount of kinetic energy available to form the weld. The required axial pressure is dictated by the materials to be welded and the interface area. The energy contained within a flywheel is determined by its mass and rotational speed.
One such inertial friction welding apparatus is disclosed in U.S. Pat. Nos. 4,702,405 and 4,735,353 as issued to Allan R. Thomson, et al. The friction welds apparatus described by Thomson is somewhat portable and utilizes a dual drive means where the second drive means includes a flywheel. In operation, the Thomson apparatus utilizes the first drives means to establish a preliminary number of revolutions per minute in the rotating workpiece before it is engaged to the stationary workpiece to which a weld is desired. Upon engagement, the spinning member begins to decelerate at a rate commensurate with the axial load and the initial revolutions per minute. Sufficient rotations of the spinning member, however, are maintained by the energy stored in the flywheel, which energy is hopefully sufficient to maintain rotational movement to overcome the initial frictional forces whereafter the first drives means maintains rotation of the spinning member until the weld is completed.
Disadvantages, however, also exist for the inertial friction weld apparatus described by Thomson. One such disadvantage is the requirement for an extremely high pressure air source and high pressure fluid flow to power the apparatus. Accordingly, the Thomson apparatus is not adapted to use pressurized air sources conventionally found at industrial facilities, but instead must utilize high pressure air supplied by special compressor units which must necessarily accompany the apparatus to the job site. This need for an additional source of pressurized air decreases to a considerable degree the portability of the Thomson system and also enhances the costs and flexibility of its operation. Moreover, the high pressure requirement also enhances the complexity of the architecture of the air motor and thus enhances the overall maintenance requirements of the system.
Other disadvantages include the requirement in the Thomson device for a flywheel to store inertial energy, which flywheel rendering the Thomson apparatus both heavy and bulky.